BACTERIA 39 



2. Animal Parasites. Useful Parasites. Many of the 

 animal parasites like lactic bacillus of the intestinal tract 

 are not only harmless but decidedly beneficial. 



Disease Forming (Pathogenic] Bacteria. The harmful 

 animal parasites are termed pathogenic bacteria. These 

 include the bacteria which cause typhus fever, diphtheria, 

 lockjaw, catarrh and cholera. Pathogenic bacteria cause 

 disease and death by the secretion of toxins, which are highly 

 poisonous substances manufactured by the protoplasm. 

 The symptoms of the disease result from the action of the 

 toxins when absorbed into the system. When the toxin 

 enters the blood stream, there is formed in the blood an 

 antitoxin or substance which counteracts the effect of the 

 toxin. If sufficient antitoxin is produced to counteract the 

 toxin completely, the patient may recover. 



A person that has recovered from a bacterial disease is no 

 longer as susceptible to the action of the bacteria causing 

 the disease because the antitoxin becomes a permanent con- 

 stituent of the blood. This lack of susceptibility is called 

 immunity. 



Frequently bacteria gain entrance to the system and, 

 unless destroyed, would multiply very rapidly in the tissues 

 and blood stream. Their presence attracts the phagocytes 

 or white blood cells, which immediately surround the bacteria 

 and digest them, or, if insoluble the bacteria are conducted to 

 and secreted by the lymphatic tissue. Some bacteria cause 

 the formation of bactericidal substances in the blood. These 

 substances destroy the bacteria, usually by dissolving them. 



Reproduction. ^Bacteria grow and reproduce very rapidly 

 because food absorption takes place through all parts of 

 their surface, which is very great in proportion to their size. 

 As soon as they reach their maximum size they must either 

 remain inactive or reproduction must occur. 



